Devices have hitherto been used for cutting a filamentous material into sections having a constant length by causing the material to pass between a bearing drum and a drum bearing blades arranged perpendicularly to the bearing drum with the cutting edges of the blades projecting radially outwardly.
However, glass slivers or threads have fairly low cohesion and, as a result, the filaments which make up the threads readily separate from one another during the operation of cutting. The thread, in fact, being wedged between the cutting edges of the blades and the surface of the bearing member, undergoes a flying-buttress effect that causes it to disintegrate. A certain number of cut filaments then rapidly form a sizable quantity of fuzz, which fills the spaces separating the blades and tends to cause irregular cutting. Solutions have been proposed for remedying this drawback as illustrated by U.S. Pat. No. 3,555,947 and French Pat. No. 1261537.
The U.S. patent discloses a blade-bearing drum cooperating with the surface of a bearing drum and equipped with projecting blades uniformly spaced on its periphery and with ejection fins placed between the blades. The ejection fins, which are capable of moving radially outwardly with their movement being limited by stops located at their two ends, eject the thread sections immediately after they are cut, by the effect of centrifugal force and thus prevent the formation of fuzz. This device yields satisfactory results provided that the thread speed is no greater than a few meters per second. For higher speeds, the centrifugal force becomes such that it often causes the breaking of the ejection fins.
The French patent discloses another device also including a blade-bearing drum. The drum is equipped with blades arranged radially and embedded in an elastomer covering which covers the entire surface of the drum. The surface of the bearing drum is coated with an elastomer that is substantially harder than the elastomer covering the surface of the blade-bearing drum. A thread is moved by being pinched between the elastomer surfaces and then brought into the cutting zone proper. The spacing between the bearing drum and the blade-bearing drum is adjusted in such a way that the elastomer of the bearing drum compresses and radially deforms the elastomer layer lining the surface of the blade-bearing drum inwardly.
This deformation has the effect of causing the cutting edges of the blades to protrude beyond the surface of the elastomer layer and thus cut the thread. Where the thread emerges from the cutting area, the elastomer lining the surface of the blade-bearing drum resumes its initial shape and ejects the section of thread. Here again the cutting speed is necessarily low due to the fact that as soon as it goes beyond a few meters per second, the rapid and violent deformation of the elastomer located on the surface of the blade-bearing drum heats the elastomer to such an extent that it rapidly degrades.
The devices described above can only cut filamentous material supplied in the form of coils of threads or slivers manufactured in accordance with classic procedures which is a costly process due to the fact that it requires the coiling and then the uncoiling of the thread. This results in sizeable handling costs and excessive thread losses because of the various handling operations.
It is consequently advantageous to produce threads in a single operation including the steps of drawing the thread from a number of forming dies and then cutting the thread into segments. This process, which is commonly called "direct cutting under spinneret" is implemented by means of a device which can perform both the functions of forming and cutting. Such a device is disclosed in French Pat. No. 2 253 718.
According to this patent, a filament is drawn by being pinched between two wheels the surfaces of which have a contact area with the first wheel, or pressure roller, having a knurled surface and the second wheel, or bearing drum, having a flexible smooth surface. Upon issuing from the contact or pinching area, the thread is driven by the bearing drum toward a cutting area formed by the contact between the drum and a third wheel, or blade-bearing drum, which is provided with a series of projecting blades mounted in slots provided in its surface.
This type of device has a number of drawbacks. In the first place, the compression which the thread undergoes between the pressure roller and the bearing drum has the effect of diminishing its cohesion, and consequently that of the cut fibers. This then results in that each cut fiber comprises a certain number of filaments that stand out to a greater or lesser extent from its surface. Cut fibers exhibiting this defect easily become caught and do not lend themselves to easy handling. Furthermore, the lack of cohesion of the fibers renders them unsuitable for certain applications.
The compresson of the thread also produces an oily deposit on the surface of the pressure roller tending to smooth it out. The thread will then tend to wind around the pressure roller a phenomenon which interrupts the operation of the device.
Whenever the rotational speed of the bearing drum and the blade-bearing drum is high, another drawback appears resulting from the impact of the blades against the bearing drum. The further out the blades project, the more difficult it is to eliminate this problem. This impact causes vibrations, the consequences of which are very rapid wearing of the blades and of the liner of the bearing drum and also a very noisy operation, which constitutes a considerable nuisance.
It is therefore an object of the invention to provide for a cutting device which, while it is capable of performing a drawing function and is fed with threads coming from one or more forming dies, also makes it possible to cut sections of the thread to a predetermined length, to eject the sections safely and completely, to counter the formation of fuzz between the blades, to prolong the life of the mechanical members of the device and to effect a substantial reduction in the noise level due to the operation of the device.